Impulse generator



June 4, i946. Wj D. HERSHBERGER 2,401,424

' I IMPULSE GENERATOR Original. Filed Sept, 30, 1937 mim/frs@ Mlcmfcmag N Y am f MIM transmitter.

Patented June 4, 1946 UNITED A STATES PATENT .OFFICE IVMPULSE GENERATOR William D. Hershberger, Haddonfeld, N. J., as- 'signor to Radio Corporation of America, a coriporation of Delaware .Original application September 3,0,.,1937LSerial No. 166,685; `lllivide'd and this application March 2 7,

19.42.,Slilfl'-N0-.436429 (Cl. Z50-36) 3'Clai1ns. ,l lV'Iy invention relates to impulsegenerators employing.thermionictubes which are used to generatevery high frequency .oscillations for very `short-periods. .More speciiically, my invention relates tothe apparatus for andthe method of generatingimpulses forradio-echo systems. l

This `application is .a .division of my application uSerial No. 166,685, filed. September 30, 1937.

:Invisible Vobjectsmay Vbe .detected by means of .radioechoes An ultra high frequencyimpulse is .and receiver are near each other, the distance in mileszfrom the transmitterof the reecting object is equal to -where T isv the interval in seconds.

In order that the transmitted impulse shall not `interfere with the received impulse, it is neces- `sarythat the interval between impulses be long enough. to permit the reflected impulse to be re- -`ceived before a second impulse is transmitted.

For example, an impulse of 2 microseconds dura- ;tion,;follvved by an .interval of v38 microseconds, aflorolsamaximum range from transmitter to reilecting object of approximately 9.1 miles. At

.greater distances, interference would result.

4-The `range of the transmitter is a function of the power radiated in an impulse. It is dicult to efciently derive the amounts of power required in the practical application of radio echo systems, which may ,operate .at .frequencies of the -orderof :500 megacycles per second. `Each `impulse will include ar thousand cycles if the frequency ispo() mega cycles .per second, and the impulse l period is f2 microseconds. -Not only are the gen- ;eratorscf relatively low .eciencyat these fre- .quenciesf'fbutthe apparatus becomes most cumbersomeionmobile operation, aircraft and the l-ilie. @One of the causes for this ineiiiciency is .the electrontransit time within the thermicnic .tubes- ,Onevof the objects of my invention is to provide .2 meansV for generating ultra high frequency currents during relatively shortald'widel'yspaced intervals. 'Y Another object is to provide means ina thermionic tube whereby the ratio oflcathde .power and plate-dissipated power greatly exceds'tlfe ratio in conventional tubes.

Another object is to provide means Within a thermionic tube whereby 'ultraihigh frequency oscillations may be .generated morefemciently than heretofore customary.

A still further object yis to provide means whereby the frequency range in .ultra high'frequency oscillators may be increased due to a'loweri'ng'of the electron transit time.`

In vdescribing my invention, reference will .be made to the accompanying drawing, in which Figure l is a schematic diagram of a radio echo system,

Figure 2 is a graphic illustration of the pulse and interval relation in the impulsegenerato of -a radio echo system, y

`Figuref is a 'view partly in section of a `conventional thermionic'tube,

Figure '4 is a view partly insection of .a thermionic tube embodied in `my invention, and

Figure 5 is a schematiccircuit diagram 'of one embodiment of the impulse generator of my in vention,

Referring'to Fig. 4l, an impulse generator or transmitter I lis connected to a dipole antenna `3 which is located within a reflector-5. -The impulse 1 istransmitte'd -in the form of a beam along the line 9 to the object Irl `from which the impulse is :reflected along 4line i3 to the receiver l5 which includes aidipole ll and a reflectorl.

Fig. 2 illustrates diagrammatically an impulse 2l vwhich Vlasts 2 microseconds; VThe impulse'2l is followed by an interval of 98 microseconds, and thereafter the impulses of 2 micresecondsand intervals of .98 microsecondsfollow each other. I have found that a .conventional thermionic tube operates at relatively-low average power output when the useful Vperiod .of oscillation is but two .percent of the total time, asis the casein radio echo systems. YI have .also found that the `eiliciency of -thermionic tubes, :designed 'for ultra high frequency oscillations, falls rapidly `as the frequency increases .due to `the-.fact that electron transit time becomes an appreciable portion of a period.

A conventional thermionic tubeior generating ultra high. frequencyV oscillations is shown in Fig. 3. Byfway of example, the tube 23 may have a cathode 25 which consumes 25-watts and an anode 3 21 which dissipates 50 watts. The rated maximum anode potential is 1250 volts. The oscillatory frequency range does not greatly exceed 200-300 megacycles per second, and at the higher frequencies the eiilciency is very low. The foregoing figures are based on continuous operation.

In Fig. 4 I have illustrated a thermionic tube which includes within its evacuated envelope 29 a cathode 3|, a grid 33, and an anode 35. The anode 35 preferably includes a number of cooling flanges 31. 'I'he envelope 29 is made larger than customary to dissipate larger amounts of heat than has heretofore been customary. The cathode or filament is of a size and of a material which will consume many times the power, and therefore liberate many times the useful emission current, usually required from cathodes, in terms of anode power dissipation.

For tubes using thoriated filament, by way of example, conventional ratios of cathode power to anode-dissipated power are from 1 to 2 to about 1 to '1 and upwards. It should be understood that the emission is proportional to the applied power for a given filament and a given temperature, vand while I shall use the term "power applied to the cathode for convenience, I am concerned with the thermionic emission.

'I'he thermionic tube of my invention uses the reciprocals of these power ratios. By way of example, the cathode power is from 2 to 7 times, and upward, the power dissipation on plate or anode. Thus, in some tubes suitable for use in my invention, I propose to derive fifty times as much instantaneous emission from the cathode as would be normally required for a given average anode current.

Conventional thermionic tubes for ultra high frequency oscillation may have, by way of example, 1000 to 1250 volts applied to the anode. The efficiency of these tubes decreases with increasing frequency so that it becomes necessary to decrease the anode potential. As the anode potential is decreased, the electron transit time increases and this further limits the frequency. 'I'hus the conventional tube becomes limited by low efliciency, and an impassable upper frequency limit.

I propose to apply ten times the anode potential. This will decrease the electron transit time by a factor equal to the square root of ten. The shorter electron transit time will increase the frequency range. Because I shall only apply the anode power for a few percent of the total time, the anode will be able to dissipate the heat over a relatively long time. 'I'he efficiency will be high during the impulse period, and therefore a large amount of energyrmay be released during the impulse time. Furthermore, since the anodD power is only applied for a small percentage of the time, the source may be designed accordingly.

An application of my invention is shown in Fig. 5. A sinusoidal wave generator 39 is connected to a saw-tooth oscillator 4|. The output circuit of the saw-tooth generator is connected to a limiter 43. The limiter output circuit includes a biasing battery 45 and a resistor 41 which are connected between the cathodes 419 of thermlonic tubes 5l 'and a movable bridge member 53. 'I'he bridge member 53 is slidably connected to the conductors of the transmission line 55 which terminates in the grid electrodes 51 of tubes 5|. The anode electrodes 59 are connected to a transmission line 6 I which includes a movable bridge member 63. The bridge member is connected to the positive terminal of a B battery E5. The Ilegative terminal of the B battery is joined to the cathodes 49. 'I'he cathodes preferably include radio frequency chokes, which may be in the form of resonant lines. The anode resonant line 6l is suitably coupled to a transmission line 61 which terminates in a dipole antenna 69. A reflector 1I may be used to focus the impulses from the clipole on the distant reflecting object.

The operation of the circuit is as follows: The thermionic tubes operate in push-pull as oscillators. Parasitic oscillations are prevented by the series resistor which prevents oscillations at a lower frequency since the tubes are effectively in parallel at low frequencies. The frequency of the desired oscillations is determined by the resonant lines.

The anode current is normally blocked by the high bias voltage applied to the grids. I'he bias voltage is decreased during the oscillatory impulse period by applying positive impulses to the grids. These positive impulses are derived by generating a current of sinusoidal wave form and of the desired frequency. The sinusoidal currents are applied to control the frequency of the sawtooth generator which establishes currents of a saw-tooth wave form. The saw-tooth currents are applied to a limiter, which only passes the positive peaks of the saw-tooth current. Since the saw-tooth peaks may be made very sharp, the periods of the impulses may be made short and the intervals between impulses long.

Thus I have described an impulse generator which includes a thermionic tube of novel characteristics. 'Ihe tube is characterized by a thermionic emission which is many times greater than previously used in tubes of equivalent anode power dissipation. The electron transit time has been greatly shortened by applying high voltages to the anode for short periods, followed by relatively large time intervals. While I have described the impulse generator in connection with a radio echo system, it should be understood that my invention may be applied to telegraphic, telephonic, or directional finding systems.

I claim as my invention:

1. In a generator which includes a thermionic tube having a grid, anode and cathode electrodes therein, said anode having a normal anode current rating, said cathode having an electron emissive capacity of the order of ten times that required to attain said normal anode current rating, and means coupling said anode to said grid to sustain oscillations, the method comprising impressing periodic potentials to said grid, wherein said periodic potentials comprise a peak value and a comparatively low value per period, said low value extending during at least ninety percent of said period, deriving from said anode peak currents exceeding said normal anode current rating by a ratio of the order of at least ten to one, and sustaining oscillations by coupling said anode to said grid.

2. A pulse generator which includes a thermionic tube wherein the variable output current thereof includes sharp peaks of which the time duration per interval does not exceed ten percent of the period of said interval, said tube including grid, anode and cathode electrodes therein, said anode having a predetermined anode current rating, and said cathode having an electron emissive capacity of the order of ten times that required to attain said predetermined anode current rating, means for deriving periodic oscillations from said anode, and means coupling said grid to said anode to sustain said oscillations.

6 emissive capacity of the order of ten times that required to attain said predetermined anode current rating, means for deriving periodic oscillations from said anode, and means for impressing upon said grid periodic potentials to sustain said oscillations.

WILLIAM D. HERSHBERGER. 

